build: Move baseline OCaml to 4.08

OCaml 4.08.0 was released on 2019-06-14, over 5 years ago.  By
requiring a slightly later OCaml version, we can drop more
compatibility code which was only used by older versions.

Consistent with qemu & libvirt, this drops support for compiling
upstream libguestfs on RHEL 8 (ocaml-4.07.0-4.el8.x86_64).

Qemu policy:
https://www.qemu.org/docs/master/about/build-platforms.html

Libvirt policy:
https://libvirt.org/platforms.html

Update the common submodule, pulling in:

  Richard W.M. Jones (4):
      qemuopts: Add ability to add raw, unquoted output to qemu scripts
      qemuopts: Fix missing break statement
      mlstdutils: Remove Option module
      Remove test for caml_alloc_initialized_string

m4/guestfs-ocaml.m4

@@ -38,10 +38,10 @@ AC_ARG_ENABLE([ocaml],
     [],
     [enable_ocaml=yes])
 
-dnl OCaml >= 4.07 is required.
-ocaml_ver_str=4.07
+dnl OCaml >= 4.08 is required.
+ocaml_ver_str=4.08
 ocaml_min_major=4
-ocaml_min_minor=7
+ocaml_min_minor=8
 AC_MSG_CHECKING([if OCaml version >= $ocaml_ver_str])
 ocaml_major="`echo $OCAMLVERSION | $AWK -F. '{print $1}'`"
 ocaml_minor="`echo $OCAMLVERSION | $AWK -F. '{print $2}' | sed 's/^0//'`"
@@ -190,21 +190,6 @@ fi
 AM_CONDITIONAL([HAVE_OCAML_PKG_OUNIT],
                [test "x$OCAML_PKG_ounit2" != "xno" && test "x$ounit_is_v2" != "xno"])
 
-dnl Check if OCaml has caml_alloc_initialized_string (added 2017).
-AC_MSG_CHECKING([for caml_alloc_initialized_string])
-cat >conftest.c <<'EOF'
-#include <caml/alloc.h>
-int main () { char *p = (void *) caml_alloc_initialized_string; return 0; }
-EOF
-AS_IF([$OCAMLC conftest.c >&AS_MESSAGE_LOG_FD 2>&1],[
-    AC_MSG_RESULT([yes])
-    AC_DEFINE([HAVE_CAML_ALLOC_INITIALIZED_STRING],[1],
-              [caml_alloc_initialized_string found at compile time.])
-],[
-    AC_MSG_RESULT([no])
-])
-rm -f conftest.c conftest.o
-
 dnl Flags we want to pass to every OCaml compiler call.
 OCAML_WARN_ERROR="-warn-error +C+D+E+F+L+M+P+S+U+V+Y+Z+X+52-3-6 -w -6"
 AC_SUBST([OCAML_WARN_ERROR])